Preparation of monocrystals of rare earth manganites



United States Patent 3,226,183 PREPARATION OF MONOCRYSTALS OF RARE EARTH MANGANITES Erwin Flix Bertaut, 29 Rue Andre Rivoire, Grenoble, France, and Francis Forrat, 2 Rue Eugene Pllsson, Marcoussis, France No Drawing. Filed June 8, 1962, Ser. No. 200,918 Claims priority, application France, June 12, 1961, 864,585 12 Claims. (Cl. 23-58) The present invention relates to the preparation of monocrystals of rare earth manganites corresponding to the general formula TMnO in which T represents a rare earth cation, which expression as used herein includes yttrium and lanthanum, as well as the elements from cerium to lutecium.

It is known that, depending upon the number of electrons contained in the N shell of the elements of the rare earth group, the manganites of such elements form two isomorphic series.

If the number of complementary 4f electrons of these elements is less than or equal to 9 (lanthanum, cerium, praseodymium, neodymium, illinium, samarium, gadolinium, terbium and dysoprosium), the manganites obtained have a crystal structure similar to the perowskites. If the number of complementary 4f electrons is greater than 9 (holmium, erbium, thulium, ytterbium and lutecium), the manganites obtained have a novel crystal structure, of a non-centrosymmetric kind on the hexagonal system, characterised by abnormal coordinances of the manganese and rare earth ions.

It is also known that the manganites prepared by the usual methods, by decomposition of manganese and rare earth nitrates and high temperature combination of the oxides obtained, yield the manganese in both the Mn and Mn forms. These compounds are semi-conductors of low resistivity, which are not suitable for use as dielectrics. It is also known that the non-centrosymmetric hexagonal crystals, when cut and metallised parallel to the base planes, can have interesting piezoelectric and ferroelectric properties.

The present invention has the object of providing a method of preparation of monocrystals of rare earth manganites grown parallel to the base planes, having a very high resistivity and interesting piezoelectric and ferroelectric properties and also articles of manufacture comprising monocrystals made by such method.

According to the invention, a method of preparing a rare earth manganite monoerystal comprises dissolving a mixture of rare earth and manganese oxides in molten bismuth oxide, supersaturating the solution by slow evaporation and then separating the rare earth manganite monocrystals formed therein.

Molten bismuth oxide used as the solvent has the advantages of readily dissolving other oxides and also of yielding monocrystals in which all the manganese is present in the form of manganites and, thus, in the Mn form.

One advantage of the method is the production of tabular monocrystals grown parallel to the base planes. The monocrystals obtained by this method have very interesting ferroelectric properties which are only slightly modified by the particular rare earth cation chosen:

( 1) The ferroelectricity only appears in the direction perpendicular to the plane of the tabular monocrystal;

(2) It is satisfactorily stable between 173 and 373 K.,

which allows use at the ambient temperature;

(3) The saturation polarisation of this temperature is about 3ac./sq. cm., the coercive field strength being about 20 kv./cm. (measurements made on samples several centimetres thick).

3,226,183 Patented Dec. 28, 1965 ice A first embodiment of the method consists in melting a mixture of bismuth oxide, manganese oxide and a rare earth oxide, at a temperature above 1000 C., and then supersaturating the solution so obtained, by slow evaporation until the first crystals appear. At this point, in order to cause the crystals to grow, slow cooling to the ambient temperature is effected, so as to avoid internal mechanical stresses.

A second embodiment of the method consists in melting a mixture of bismuth oxide, manganese oxide and a rare earth oxide, at a temperature above 1000 C., and then supersaturating the solution so obtained, by slow evaporation until the bismuth oxide completely disappears. The mixture is then uniformly cooled to the ambient temperature.

Various examples, given by way of illustration, are described below. The details described in these examples should be considered as comprising part of the invention, though all equivalent means can also be used without exceeding its scope.

Example 1 A homogeneous mixture constituted by 30 gram moles of Bi O 6 gram moles of Mn O and 3 gram moles of Y O was melted in a platinum crucible at 1350 C. The solution was supersaturated by slow evaporation at 1410 C. until the Bi O was completely eliminated, according to the second embodiment of the method. The solution was then slowly cooled to room temperature at a rate of 10 C./h., to produce a crop of yttrium manganite crystals.

Example 3 A homogeneous mixture constituted by 20 gram moles of Bi O 4 gram moles of Mn O and 3 gram moles of Er O was melted in a platinum crucible at 1350 C. The solution was supersaturated by slow evaporation at 1450 C. until the Bi O was completely eliminated, according to the second embodiment of the method. The solution was then slowly cooled to room temperature at a rate of 10 C./h., to produce a crop of erbium manganite crystals.

We claim:

1. A method of preparing rare earth manganite monocrystals, which comprises dissolving a mixture of a rare earth oxide and manganese oxide in molten bismuth oxide, supersaturating the solution by slow evaporation of the bismuth oxide and then separating the rate earth manganite monocrystals formed therein.

2. A method as claimed in claim 1, in which the monocrystals are separated by slowly evaporating the bismuth oxide until the first rare earth manganite crystals appear and then slowly cooling the solution to room temperature at a rate of from 4 C. to 10 C. per hour.

3. A method as claimed in claim 1, in which the monocrystals are separated by the complete evaporation of the bismuth oxide.

4. A method of preparing thulium manganite monocrystals comprising the steps of melting a homogeneous mixture comprising 15 gram moles of B-i O 3 gram moles of Mn O and 3 gram moles of Tm O at 1350 C., supersaturating the solution by slow evaporation of bismuth oxide at 1400 C. until the first crystals appear andthen slowly cooling to room temperature.

5. A method as claimed in claim 4, in which the solution is cooled to 1200 C. at a rate of 4 C./h. and from 1200" C. to room temperature at a rate of 10 C./h.

6. A method of preparing yttrium manganite monocrystals comprising the steps of melting .a homogeneous mixture comprising 30 gram moles of Bi O 6 gram moles of Mn O and 3 gram moles of Y O at 1350 C., supersaturating the solution by slow evaporation at 1450 C. until the Bi O is completely eliminated and slowly cooling the mass to room temperature.

7. A method of preparing erbium manganite monocrystals comprising the steps of melting a homogeneous mixture comprising 20 gram moles of Bi O 4 gram moles of Mn O and 3 gram moles of Er O at 1350 C., supersaturating the solution by slow evaporation at 1450 C. until the Bi O is completely eliminated and slowly cooling the mass to room temperature.

8. A method as claimed in claim 6, in which the mass is cooled at a rate of 10 C./h.

9. A method as claimed in claim 1 in which the dissolution of the oxides is effected at a temperature greater than 1000 C.

4 10. A method as claimed in claim 9 in which the temperature is about 1350 C.

11. A method as claimed in claim 1 in which the molar ratio of bismuth oxide to manganese oxide is about 5 to 1. 12. A method according to claim 7 in which the mass is slowly cooled at a rate of about 10 C./h.

References Cited by the Examiner UNITED STATES PATENTS 2/1963 Remeika 252 62.5 8/1963 Stuijts 252-62.5

OTHER REFERENCES Jonker et al.: Phillips Technical Reviewf vol, 18, No. 6, November 1956, pages 145-180.

Mones et al.: J. Phys. Chem. Solids," Pergamon Press, 1958, vol. 4, pages 217-222.

Sitova: Soviet Physics, Solid State, 1960, pages 1714- 15. Translated from Fizika Querdogo Tela, vol. 1, No. 12.

BENJAMIN HENKIN, Primary Examiner. 

1. A METHOD OF PREPARING RARE EARTH MANGANITE MONOCRYSTALS, WHICH COMPRISES DISSOLVING A MIXTURE OF A RARE EARTH OXIDE AND MANGANESE OXIDE IN MOLTEN BISMUTH OXIDE, SUPERSATURATING THE SOLUTION BY SLOW EVAPORATION OF THE BISMUTH OXIDE AND THEN SEPARATING THE RATE EARTH MANGANITE MONOCRYSTALS FORMED THEREIN. 